As the demand for rubber products increase, there is a need for improved processes for mixing rubber compounds that provide reduced mixing time and thus improve mixing capacity in a cost-effective manner without compromising the physical properties of the final products. In addition, increasing demands for difficult-to-mix compounds also require processes allowing increased mixing capacity. These difficult-to-mix compounds include rubber blend compositions containing high natural rubber content with polybutadiene or styrene butadiene rubber elastomers and filled with reinforcing small particle carbon black. The finer grade carbon blacks are often very difficult-to-mix. Many products, such as higher mileage truck tires, are made from rubber compounds including finer grade carbon blacks and other difficult-to-mix rubber materials, underlining the importance of improved rubber mixing processes.
Improvements in mixing efficiency can offer significant cost savings in the production of rubber compounds. Mixing fine particle carbon black into various grades of rubber and rubber blends comprising a tougher natural rubber and a softer synthetic diene elastomer can be a difficult and time consuming process. Often, long mixing times or multistage mixes are required to produce compounds which can be handled adequately on down-stream processing equipment. In some cases, pre-mastication of the natural rubber is helpful, but pre-mastication may result in a loss of mixing capacity as time in the mixer is dedicated to pre-mastication and not to productive mixing. In the production environment, the Mooney viscosity of the compound gauges the quality of the mix. Only upon achieving a predetermined Mooney viscosity does further downstream processing continue (e.g., calendaring and extrusion). Often these difficult-to-mix compounds require several passes through a mixer in order to achieve the desired viscosity. While pre-mastication techniques and additions of oils or soaps may reduce compound viscosities, physical and dynamic mechanical properties may suffer when theses additives are included.
Techniques to reduce the viscosity of a natural rubber blend compound include masticative reduction of molecular weight, chemical peptization (chemical-oxidative molecular weight reduction), addition of diluents and lubricants (soaps and oils) and lowering the filler loading in the rubber blend compound. However, each of these techniques has limitations or some potential disadvantage. Mastication and chemical peptizing result in a disadvantage of increased mixing time. Processing aids and peptizers result in compounds with lower Mooney viscosity and processing improvements, but the final vulcanizates often suffer from reduced modulus and increased heat buildup, reducing the compound performance in dynamic applications. Other processing aides such as soaps and oils may provide some lubricity and softening activity. Peptizers enhance the oxidative degradation of natural rubber during the mixing process. Chemical peptizing at high temperatures leads to increased amounts of low molecular weight polymer due to the random nature of the oxidative scission process. The low molecular weight polymer adversely affects dynamic mechanical properties. Likewise the use of diluents, adding soaps and oils, also reduce the viscosities, but may result in the deterioration of dynamic mechanical properties in proportion to the levels of diluents used. Compound modification by lowering the filler loading can only be used when the demands of the formulation have sufficient tolerance to afford such changes.
Antidegradants are preferably quinones, quinone diimines or quinone imines, such as N-(1,3-dimethylbutyl)-N′-phenyl-p-quinonediimine, may protect natural rubber from oxidative degradation during the mixing process. These antidegradants, particularly quinone diimines (QDI), are multi-functional chemicals which function primarily as a long-lasting antidegradant. QDI enhances mixing and provides viscosity reductions in natural rubber compounds with little or no loss in vulcanizate performance by protecting the polymer from excessive molecular weight reduction during mixing while providing both bound antioxidant and diffusible antiozonant activity in the final vulcanizate. QDI is extremely efficient at capturing chains broken during the shearing action of the mixing process. Published patent application WO 99/20687, filed 19 Oct. 1998 by Ignatz-Hoover, describes the high temperature mixing of elastomeric rubber materials with QDI and carbon black prior to vulcanization and published patent application WO 01/92423, filed 22 May 2001 by Lamba et al. describes compositions containing carbon black and a QDI compound. Accordingly, a quinone diimine antidegradant typically reacts faster than conventional antidegradants to stabilize broken chains in natural rubber and minimize the oxidative chain degradation that occur during mixing of natural rubber compounds in intensive mixing equipment. The quinone diimine is believed to react by adding to the radical chain end of natural rubber during mixing. This reaction not only produces a polymer-bound PPD moiety, but prevents the recombination of the broken chains. These two benefits accelerate viscosity reduction and improve polymer-to-filler interaction during mixing of natural rubber compositions that do not include synthetic non-isoprene rubber elastomers.
While it is known that free radical chemistry is the origin of molecular weight reduction reactions in natural rubber compounds, it is also known that gel formation in butadiene-based synthetic rubber elastomers results from radicals formed during mixing. In the butadiene-based elastomers, radicals are believed to react to increase molecular weight thereby increasing viscosity, or upon further reaction, forming phases of crosslinked rubber either free or in association with carbon black (i.e. bound rubber formation). Therefore, what is needed are improved methods for mixing rubber blends containing natural rubber and butadiene-containing rubber elastomers, such as styrene-butadiene copolymers or styrene-butadiene-isoprene copolymers, with a desired reduction in viscosity and a desirably low discharge temperature, thereby providing reduced mixing times and increased mixing capacities.